The present invention relates to a method and apparatus for promoting uniform processing within a substrate processing tank. More specifically, the present invention relates to a method and apparatus for controlling fluid temperatures and chemical concentrations within a tank used for semiconductor processing, such as a megasonic tank.
In the field of semiconductor processing, it is important to ensure that each processed wafer (e.g., patterned, or unpatterned, etc.) experiences identical processing conditions so as to ensure consistent quality. Bath-type processes present particular process uniformity issues. Evaporation, chemistry decomposition and process temperature each vary over time. For example, conventional megasonic tank-type cleaners submerge a wafer in a tank of cleaning chemistry, typically comprising a mixture of deionized water, ammonia and hydrogen peroxide. To achieve desired cleaning performance, the concentration of each component is carefully controlled as the tank is filled with chemistry. However, as ammonia quickly evaporates, and hydrogen peroxide readily decomposes (forming water and oxygen) the concentration of each component immediately begins to change.
Further, the chemistry is heated to a desired temperature as the chemistry flows into the tank. To achieve desired cleaning performance and ensure consistent quality wafers, the chemistry is maintained within a close tolerance of the desired temperature. However, during processing, the heat generated by the transducers which megasonically energize the chemistry also undesirably heat the chemistry. This undesirable heating of the chemistry not only causes processing variations, but also further accelerates the rate of chemical evaporation and decomposition.
To combat the process drift described above, test wafers are periodically processed in a megasonic tank, and the chemical concentration of the bath is adjusted based on the cleanliness of the test wafers. This method is labor intensive and subject to human error. Moreover, system productivity is decreased by the need to run test wafers. Alternative methods continuously bleed chemistry from the tank while feeding fresh chemistry to the tank. These methods require huge energy consumption to heat the continuous volume of chemistry flowing to the tank.
Accordingly, a need exists for an improved method and apparatus for controlling temperature and concentration of chemistries within a substrate processing tank such as a megasonic tank.
The present invention provides an improved method and apparatus for adjusting chemistry concentrations and temperatures within a substrate processing tank. In a first aspect the inventive method comprises checking the fluid level within the tank, and, if the level is higher than a predetermined upper level, bleeding an amount of fluid from the tank; if the level is lower than a predetermined lower level, flowing an amount of fluid to the tank, and if the level is between the predetermined upper and lower levels, bleeding an amount of fluid from the tank and flowing an amount of fluid to the tank.
In a second aspect, the inventive method comprises flowing water into the tank at a flow rate at least equivalent to the flow rate of water required to achieve a chemistry spike of a predetermined concentration and volume (e.g., 50 ml/minute water for a 210 ml/minute flow rate of 5 parts water and 2 parts chemical), prior to beginning the flow of chemicals.
In a third aspect, the invention provides a method and apparatus for heating or cooling chemistry to a predetermined temperature as the chemistry is recirculated.
Each aspect of the invention may be automated, in the form of a program product, and therefore may reduce the labor costs associated with many conventional methods. Each aspect also may exhibit its own advantages, for instance the first aspect of the invention may conserve both energy and chemistry, as chemistry is only periodically flowed into the tank, requiring less chemistry and less energy to heat and pump the chemistry. The second aspect may prevent harmful concentrations of chemistry from being released, and the third aspect may reduce evaporation and decomposition rates. Each aspect of the invention may reduce process drift, increase processing uniformity and the uniform quality of wafers processed thereby.
Other features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings.